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A friend showed me this progression:

G7, F#m7b5, B7, Em7, A7, Dm7, G7, CMaj7, Cm7, Bm7, Bdim7, Am7, Ab7, Bm7, Bb7, Am7, Ab7

For the most part, I understand that at its heart it is built on a I vi ii V in the key of C.

I do not understand some of the chords though. What does the F#m7b5 function as? What about that B7? Also, the Cm7, Bm7, Bdim7 sounds so good to my ear but what is the theory behind that?

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  • F#m7b5 -> B7 -> Em7 is ii-V-I minor progression in key of E-minor

then

  • Em7 -> A7 can be thought as ii-V7 in Dmajor or Em7 -> A7 -> Dm7 as D minor ii-V7-I
  • Dm7 -> G7 -> Cmaj7 is normal ii-V7-I in key of C major

then

  • Cm7 shifts key to C minor and begins chromatic root movement downwards with Cm7 -> Bm7 -> Bdim7 -> Am7 -> Ab7
  • last four chords is repetition of this root movement with variation Bdim7 became Bb7 (could be also Bbm7)

in this last descending chords all of them are passing chords leading to Ab7 (which could be think as a leading chord to G7 if You cycle through this progression)

In Jazz there is a common:

  • substitution with so called tritone or flat five dominant chord (will be Bb7 in Your progression)
  • or diminished 7th chord with same root or root half step below (Your Bdim7 chord) as a passing chord.

So this piece shifts a key centers a lot :)

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  • Ah I should have considered the key shifts. I have some knowledge of the tritone sub and the dim7 as a passing but near enough to break down this progression.
    – Robb
    Commented Sep 21, 2011 at 14:04
  • Tritone subs are typically not diminished 7ths. Diminished chords in jazz (built from the whole/half diminished scale) typically function as rootless 7b9s. For example, Bdim7 and Bb7b9 are functionally the same chord (a tritone sub for E7, the V of Am7), thus the sub. The diminished chord is typically used to provide linear root movement.
    – Rein Henrichs
    Commented Sep 21, 2011 at 17:29
  • @Rein, yes I know that. My comment apparently reads incorrectly.
    – Robb
    Commented Sep 21, 2011 at 17:37

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